COMPACT BIOFILM REACTOR FOR AEROBIC WASTEWATER TREATMENT Dan Dickeson, Lantec Products Inc.* Toshiki Yoshimura, Able Company Ltd. *Lantec Products Inc., 5308 Derry Ave. Unit E, Agoura Hills, CA 91301 ABSTRACT Continuously self-cleaning fixed-film (CSCF) bioreactors for BOD reduction use high-efficiency ran- dom packing. They are compact units of simple design, requiring minimal operator attention or mainte- nance. Wastewater is aerated and mixed by an air-lift system in a reactor vessel packed with polypropy- lene media which breaks the air stream into fine bubbles for efficient oxygen transfer. The packing media has a high surface area, with uniformly spaced rods or strands for easy dislodging of accumulated biofilm. The media is cleaned at intervals by a continuous, rotary air-shearing system to control the biofilm thickness and age distribution, and prevent development of anaerobic conditions. The microbes remain in a high-growth phase, but they form a fully developed food chain, in which larger organisms consume small- er ones. The result is more BOD conversion to water and CO2, and less to biomass, so sludge production is minimized. The acclimated biofilm is resistant to shock loading. The frequency of air shearing can eas- ily be adjusted for optimum performance. Depending on the application, CSCF bioreactors may be used alone, or as a cost-effective pre- treatment stage to upgrade existing treatment systems. A pilot reactor 900 mm in diameter and 2.7 m tall was recently installed at a winery whose existing aeration lagoons were becoming overloaded. After a 3-week startup period, the pilot unit reduced average BOD levels from 2,370 mg/L to 326 mg/L even without clarification. When a coagulant was added to the treated water and suspended solids settled out, the BOD fell to 11.8 mg/L. The unit destroyed 2.2 kg of BOD per day per cubic meter of reactor volume, compared with 0.7 kg/day·m3 for a typical activated-sludge process. Keywords: reactor aerobic wastewater fixed film continuous cleaning. INTRODUCTION Biochemical oxidation is an effective means of removing many organic contaminants from waste- water, but conventional aerobic treatment systems have a number of drawbacks. Trickling filters take up too much space, and tend to cause secondary pollution such as odor and flies. Activated sludge process- es generate large amounts of biosiolids, and require careful monitoring because they are susceptible to shock caused by sudden changes in loading. Rotating biological contactors are hardier and more compact, but they are expensive and prone to mechanical problems. Reactors using fixed submerged media per- form well at low loadings, but they are easily plugged by excessive buildup of biomass. These problems have been overcome by means of a continuously self cleaning fixed-film (CSCF) bioreac- tor. Biofilm thickness is controlled by periodic cleaning of the media without interrupting the operation. A simple design minimizes the need for operator attention and equipment maintenance. METHODOLOGY The operation of a CSCF bioractor is illustrated in Figure 1. Figure 1. Cross Section of CSCF BioReactor using No. 4 NUPAC® Media Wastewater enters the top of a reactor vessel packed with No.4 NUPAC® , where it is aerated and mixed by an air-lift system. The packing breaks the air stream into fine bubbles, resulting in oxygen trans- fer so efficient that the system uses only about half the air of conventional fixed-film reactors. NUPAC® has a surface area of 125 m2/m3, with uniformly spaced polypropylene rods for easy dislodging of accumulated biofilm. The packed section of the reactor is rotated slowly over an air-shearing device which periodically strips off most of the accumulated biomass. This makes it possible to control the biofilm thickness and age distribution, and prevent the development of anaerobic conditions. The frequency of air shearing is opti- mized so that the microbes remain in a high-growth phase, but they still form a fully developed food chain, in which larger organisms consume smaller ones. The result is more BOD conversion to water and CO2, and less to biomass, so sludge production is minimized. Once acclimated, the biofilm is highly resistant to shock caused by abrupt changes in BOD loading or exposure to toxins. Long-term variations in loading can be compensated for by adjusting the frequency of air shearing. For large reactors, it is easier to make the packed section stationary, with an air-shearing device that rotates underneath it. The equipment can be adapted for installation in existing concrete basins. For applications in which the wastewater contains high levels of grease or other contaminants which tend to stick to the media, CSCF bioreactors can be filled with a special packing, known by its Japanese nickname Jiraiya. This media has the structure of a bottle brush, with long polypropylene strands held in place by a stainless steel core. The flexible strands bend back and forth during air shear- ing, for easier dislodging of sticky residues. RESULTS A pilot reactor was installed to treat beverage processing wastewater at the Suntory winery north of Tokyo, Japan. The winery generates 2,000 m3/day of wastewater, and expanded grape pressing oper- ations were projected to increase the influent BOD from 1,500 mg/L to 3,000 mg/L, overloading the exist- ing aeration lagoons. There was not enough space for additional lagoons, nor capacity to handle twice as much sludge. It was proposed to use a CSCF bioreactor as a pretreatment stage, as shown in Figure 2. Figure 2. CSCF Bioreactor used as Pretreatment Stage for Activated-Sludge Process The fixed-film reactor would reduce the loading on the existing system, insulate it from shock due to loading fluctuations, and minimize the increase in sludge production. The pilot plant unit was 900 mm in diameter and 2.7 m tall. It processed a slip stream of 1,600 L/day. After a 3-week startup period, the pilot unit reduced average BOD levels from 2,370 mg/L to 326 mg/L even without clarification. When coagulant was added and suspended solids settled out, the BOD fell to 11.8 mg/L. The unit destroyed 2.2 kg of BOD per day per cubic meter of reactor volume. That compares with 0.7 kg/day·m3 for a typical activated-sludge process. The pilot plant results were scaled up to design a system of 3 CSCF bioreactors, each 7.5 m in diameter and 6 m high. Those can destroy as much BOD as two 5,000-m3 aeration lagoons, in a fraction of the space. The simple carbohydrates in beverage processing wastewater are among the most readily biodegradable contaminants. A more challenging application was the Nippon Ham sausage plant in Ono City, Hyogo Prefecture, Japan. This was a greasy wastewater stream with considerable variability, which had to be treated to a level that would allow discharge to the city sewer without further treatment. A system of 3 large CSCF bioreactors was installed, each 3.8 m in diameter and 5 m high. Two of the units operate in parallel, in order to provide more residence time in a high-rate first stage. The design parameters and actual performance of the system were as follows. Although biochemical oxidation is most often used to treat food and beverage processing waste- water, CSCF bioreactors have also been used effectively for synthetic contaminants. A Fuji Paperboard plant in Saitama Prefecture, Japan generates wastewater from corrugated manufacturing lines. The BOD in the water includes not only starches, but also adhesive residues includ- ing vinyl acetate and polyvinyl alcohol, which are relatively difficult to degrade biologicallly. The waste- water composition is highly variable, so an activated-sludge process would be prone to shock due to changes in loading. A single reactor 2 m in diameter and 5 m high was installed at the plant to treat 60 m3/day of waste- water. It reduces average influent BOD levels of 800 mg/L to 20~40 mg/L even without clarification. That works out to 3.0 kg of BOD per day per m3 of reactor volume. DISCUSSION The CSCF bioreactor was developed in Japan, where its compact design is a key advantage due to high land costs. Other noteworthy features have become apparent as this technology is applied to a growing range of applications. Continuous operation with periodic renewal of the biofilm layer makes it possible to maintain a more diverse microbial population than in either suspended-growth processes or in fixed-film reactors operated batchwise on the same wastewater. This is particularly helpful if the contaminant mix and/or tem- perature of the wastewater tends to vary from day to day, or from one shift to the next. CSCF bioreactors work particularly well on wastewater streams which tend to promote the growth of filamentous organisms. Those microbes do not cause sludge bulking problems, because they are agglomerated with other organisms on the packing media, and get removed during air shearing as large particles, rather than individual suspended cells. The biofilm layer thickness and cell age distribution can be fine tuned by simply resetting the air shearing frequency on the control panel. Operating costs for CSCF bioreactors are low. The continuous self-cleaning feature makes it pos- sible to use packing media having high specific surface area. This results in very efficient oxygen trans- fer to the water, minimizing the air flow required, so that the blower consumes less power. Air lift provides all the agitation needed, and the motor used to rotate the air-shearing tube consumes a fraction of a kilo- watt. The inherent simplicity of the design minimizes maintenance requirements. Sludge production is often low enough to permit direct discharge of the effluent without clarifica- tion. It has even been proposed to use CSCF bioreactors in place of dissolved-air flotation equipment in some applications, in order to reduce sludge volume and chemical consumption. Research is currently underway to adapt CSCF bioreactor technology for use with specially devel- oped microbial populations, including for nitrogen and phosphorus removal. CONCLUSIONS Continuously self-cleaning fixed-film bioreactors are simple, compact devices which maximize the inherent advantages of attached-growth processes for aerobic BOD reduction.
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